1. Technical Field
The invention relates to a light modulation element, its driving method and a driving apparatus.
2. Related Art
Various rewritable marking techniques which are highly convenient have been researched. As one direction, in recent years, attention has been focused on a display element using cholesteric liquid crystal because it has features such as a memory property capable of retaining display without power supply, capability of providing bright display because of using no polarizing plate, and capability of producing color display without using any color filter.
A planar texture exhibited by cholesteric liquid crystal (chiral nematic liquid crystal) causes a selective reflection phenomenon to occur in which light incident in parallel with a helical axis is separated into right optical rotation and left optical rotation, a circular polarization component matching a twist direction of the helical axis is Bragg-reflected, and the remaining light is allowed to pass through. Let a helical pitch be p, average refractive index in a plane orthogonal to the helical axis be n, and complex refractive index be Δn, center wavelength λ of reflected light and reflection wavelength width Δλ are represented as λ=n×p and Δλ=Δn×p, respectively. Reflected light caused by the cholesteric liquid crystal in the planar texture exhibits vivid color dependent on the helical pitch.
The cholesteric liquid crystal having positive dielectric constant anisotropy exhibits three states of (i) a planar texture in which the helical axis becomes perpendicular to the cell surface and the selective reflection phenomenon is caused to occur for incident light as shown in FIG. 10(A), (ii) a focal conic texture in which the helical axis becomes almost parallel to the cell surface and incident light is allowed to pass through while it is forward scattered a little as shown in FIG. 10(B), and (iii) a homeotropic texture in which the helical structure comes loose and liquid crystal director is oriented in the electric-field direction for allowing incident light to pass through almost completely as shown in FIG. 10(C).
Of these three states, the planar texture and the focal conic texture can exist bistably without electric field. Therefore, the texture state of the cholesteric liquid crystal is not uniquely determined with respect to an intensity of the electric field applied to a liquid crystal layer. If the planar texture is an initial state, the state changes in order of the planar texture, the focal conic texture, and the homeotropic texture with an increase in intensity of the electric field. If the focal conic texture is the initial state, the state changes in order of the focal conic texture and the homeotropic texture with an increase in intensity of the electric field.
On the other hand, if the intensity of the electric field applied to the liquid crystal layer is set to zero suddenly, the planar texture and the focal conic texture maintain the state intact and the homeotropic texture changes to the planar texture.
Therefore, the cholesteric liquid crystal layer just after a pulse signal is applied thereto exhibits switching behavior as shown in FIG. 11. When a voltage of the applied pulse signal is Vfh or more, the cholesteric liquid crystal enters the selective reflection state in which the homeotropic texture has changed to the planar texture. When the voltage is between Vpf and Vfh, the cholesteric liquid crystal is in a transmission state due to the focal conic texture. When the voltage is Vpf or less, a state of the cholesteric liquid crystal before the pulse signal is applied is continued, namely, the cholesteric liquid crystal is in the selective reflection state due to the planar texture or the transmission state due to the focal conic texture.
In FIG. 11, the vertical axis is normalized light reflectivity. The light reflectivity is normalized so that the maximum light reflectivity is set to 100 and the minimum light reflectivity is set to 0. Since a transition region exists among the states of the planar texture, the focal conic texture, and the homeotropic texture, the case where the normalized light reflectivity is 50 or more is defined as a selective reflection state and the case where the normalized light reflectivity is less than 50 is defined as a transmission state. Also, a threshold voltage of texture change between the planar texture and the focal conic texture is denoted by Vpf, and a threshold voltage of texture change between the focal conic texture and the homeotropic texture is denoted by Vfh.
FIG. 12 is a drawing to schematically show a state in which an image is written to a display element with a light irradiation device. As shown in FIG. 12, in the display element a display layer, which is a liquid crystal layer, and an organic photosensitive layer, which is a photoconductive layer, are deposited between a pair of transparent electrodes (with sandwiching a shading layer (not shown), if necessary). The photoconductive layer consists of a charge generation layer, a charge transportation layer and another charge generation layer, and the charge generation layers sandwich the charge transportation layer therebetween. Also, a pair of substrates sandwich the display layer, the photosensitive layer and the transparent electrodes. The surface on a side of the organic photosensitive layer is exposed to image light by an exposure device in a state where a certain voltage is applied to both the transparent electrodes. Thereby, any desired record image can be written therein.
With this display element, if units in each of which the display layer and the photoconductive layer are sandwiched between the electrode layers are deposited as three colors of R, G, and B, a full color image can also be formed.